Hitherto, polyethylene terephthalate (PET) film substrate coated with magnetic layer has been widely used as a magnetic recording medium. However, PET film is inferior in water resistance, and sometimes it can not be used because it may be hydrolyzed at high temperature under high humidity. Glass transition temperature of PET is 60.degree. to 80.degree. C. Elastic modulus may be sharply decreased at a temperature higher than the above range, and elongation or the like may readily occur. Accordingly, reliability is not always sufficient. Magnetic tape using such PET film as a base film has problem, that is, bad reproduction due to elongation of the base film at high temperature, for example, in a car.
As a base film for magnetic disk such as floppy disk demand for which has been increasing, the above PET film has been used. Recently, as miniaturization and densification of floppy disk, further improvement of physical properties such as heat resistance, moisture resistance, dimensional stability and the like has been required. However, a magnetic disk using the above PET film as a base material has problems in dimensional change with change of moisture, anisotropism of thermal expansion coefficient in face, eternal shrinkage when left at high temperature under high moisture over a long period of time. Polyphenylene sulfide (PPS) film used as a base film for this magnetic disk may overcome the above problems, but it is too expensive for practical use. Satisfactory base materials for magnetic disk have not been proposed, yet.
On the other hand, magnetic recording medium has been further densified, and demand for its reliability has been increasing. One of the means for densification is a method wherein a magnetic layer is laminated on a base film by a vapor deposition method. This method, however, has problems, for example, insufficient heat resistance, separation of oligomer when PET film is used as a base film.
Further, demand for magnetic card has been increasing. As base films for magnetic cards, polyvinyl chloride film and polyester film have been known. However, base film of polyvinyl chloride has low heat resistance and less resilient. Base film of polyester has problems in practical application, for example, insufficient whiteness, too much gloss. Base films for magnetic cards which are excellent in heat resistance, chemical resistance, electrical insulating properties, dimensional stability and the like and have sufficient whiteness are required.
As described above, the magnetic recording medium obtained from the conventional base film have various problems for practical application, for example, insufficient sliding properties, smoothness, heat resistance, moisture resistance, dimensional stability and the like.